1. Field of the Invention
The present invention concerns travelling-wave tubes with operation in wide bandwith and at high peak power and/or mean power levels.
A travelling-wave tube is formed by the association of a long, thin electron beam and a structure, such as a delay line, designed to guide a microwave that has to be amplified. This delay line is quite often made with a thin metal strip, with a substantially rectangular cross-section, helically wound in forming non-contiguous turns. Thus a helix is obtained.
This helix is held, centered by dielectric rods, in a metal sleeve that forms the body of the travelling-wave tube. The number of rods is generally greater than or equal to three.
2. Description of the Prior Art
In the prior art, the helix is made with a thin, substantially rectangular-sectioned strip of a metal that is a good conductor of heat and electricity. The helix is brazed to the rods which are themselves brazed to the inside of the sleeve. The helix is made of copper for example. Quite often, a sleeve, also made of copper, and rods made of beryllium oxide are used. Since copper has a low electrical resistivity, the Joule's heat losses will be low. Owing to the high thermal conductivity of copper, at the brazing joints efficient heat dissipation is obtained along the helix and the sleeve. The heating of the helix will be reduced to the minimum and will enable operation and high peak power and/or mean power values. This kind of a structure may be used in wide-band travelling-wave tubes.
This structure nevertheless has one drawback. There are inevitably brazing beads at the helix/rod interface. If a brazing is to be reliable, it is preferable that an accumulation of brazing material or bead should go over on either side of the turn of the helix. Each bead is in contact with one side of the turn and with the rod.
During pulsed operation at very high power, due to the needle effect (by which electrical charges accumulate at the tip of a pointed body that is electrified), electrical arcs might be created between two facing beads located on two consecutive turns.
Travelling-wave tubes provided with such helical windings are therefore limited in terms of peak power value.